Magnetic thin films generally include a substrate material and at least one layer of magnetic material supported by the substrate material. Magnetic thin films are used in a variety of applications, including information storage and microwave and millimeter wave devices. For example, disks, disk drives, and tape incorporating magnetic thin film media are efficient information storage means.
In all applications the need for magnetic thin films having improved magnetic properties has accelerated. For example, in the field of information storage there is an accelerating demand for magnetic recording media having improved data storage capacity, lower noise, and reduced cost. Science and industry have endeavored to address this demand by developing magnetic recording media having better-defined grain structures, increased recording densities, and lower associated noise. However, the relatively recent explosion in the use of personal computers in the office and at home has kindled a demand for magnetic recording media with even greater data storage capacity, lower noise, and reduced cost.
In the information storage field, data is commonly stored on magnetic thin film media in the form of magnetic disks. Most magnetic disks may be broadly classified as either flexible ("floppy") or rigid ("hard"). Binary information is stored on the disks in magnetic bits within segmented circular tracks partitioned on the magnetic surface of the disk. A typical disk drive includes one or more of magnetic disks rotated on a central axis. To either supplement or retrieve information stored on a disk, a magnetic head, or slider, is disposed on a movable arm positioned over and very close to the surface of the disk. The head passes over the disk's segmented tracks and accesses or adds to the information stored on the disk as the disk rotates.
A typical magnetic disk includes one or more thin film layers disposed on a substrate material such as, for example, aluminum or glass. In their basic form, magnetic disks include a magnetic layer and, typically, an overcoat to protect the magnetic layer. The overcoat also may be coated with an organic lubricant. An overcoat is always used on cobalt-based thin films because such films easily oxidize when exposed to air and are not particularly wear resistant. The magnetic layer is the primary element of the magnetic disk on which information is stored; by inducing magnetic fields within particular regions of the magnetic layer on the disk, bits of information are recorded. Because information is stored on the disks in this way, the magnetic film's magnetic properties, such as coercivity (H.sub.c), remnant magnetization (M.sub.r), coercivity squareness (S*), and switching field distribution, directly influence the media's recording performance. These and other magnetic properties of magnetic disks, tape, and other thin film magnetic recording media depend in large part on the microstructure of the magnetic film layer.
One family of magnetic thin films are the ferrite thin films. The magnetic ferrite thin films include a crystalline magnetic layer composed of one or more ferrite compounds such as, for example, strontium ferrite, barium ferrite, zinc ferrite, manganese ferrite, ferrous ferrite, cobalt ferrite, nickel ferrite, magnesium ferrite, cadmium ferrite, copper ferrite, lithium ferrite, and lead ferrite, as well as mixed ferrites made of combinations of these and other ferrites, including garnets. Certain of the ferrite thin films have applications as the magnetic layer in thin film magnetic recording media. However, ferrite thin films may be used in a variety of other technologies, including microwave and millimeter wave devices.
Thin films fabricated from the ferrite compound barium ferrite and/or strontium ferrite (also known as barium and/or strontium hexaferrite) are known for use as thin film magnetic recording media. In their crystalline form, barium ferrite and strontium ferrite have the compositions BaFe.sub.12 O.sub.19 (BaO.multidot.Fe.sub.2 O.sub.3) and SrFe.sub.12 O.sub.19 (SrO.multidot.6Fe.sub.2 O.sub.3), respectively. The magnetic properties of barium ferrite and strontium ferrite differ significantly from those of the cobalt-based alloys commonly used as the magnetic layer in thin film magnetic recording media. In their crystalline state, barium ferrite and strontium ferrite have lower saturation magnetization (M.sub.s), but a much higher anisotropy field (H.sub.k) than typical cobalt-based alloys. Considered from the viewpoint of optimizing the media's magnetic properties and ignoring any limitations of magnetic head technology, although reduced M.sub.s is not optimal because it suggests a corresponding decrease in recording signal output, high linear densities, which require narrow transition width, may be achieved with barium ferrite and/or strontium ferrite. Transition width is believed to be proportional to the square root of M.sub.r t/H.sub.c if S* is .congruent.1 and the film thickness t is much smaller that the head-to-media spacing. Barium ferrite and strontium ferrite are, therefore, favorable in terms of transition width because of their relatively low M.sub.s and high H.sub.c. Magnetic layers of barium ferrite and strontium ferrite also exhibit excellent chemical stability against environmental corrosion, and the layers' high mechanical hardness may make an overcoat layer unnecessary.
For ferrite thin film longitudinal magnetic recording media, including barium ferrite and strontium ferrite media, the desired crystalline structure, or texture is that wherein the c-axes, which are also the magnetic easy axes of the grains, are oriented in the film plane. In practice, current technology limits optimal grain texture to that in which the c-axes of the grains are randomly oriented. Theoretically, higher values of H.sub.c and S* (.congruent.1) narrow the transition width and thereby improve recording performance. Practically however, the M.sub.s of the writing head is a limiting factor for the suitable upper limit of H.sub.c of the media. The current limitations of magnetic head technology may make it necessary to reduce the H.sub.c of the media to a range that will allow current-generation write heads to write to the media (currently less than about 3000 Oe). Known techniques for reducing barium ferrite media H.sub.c include, for example, doping the media's magnetic film with appropriate amounts of dopants such as, for example, cobalt, titanium, and zinc in order to reduce the anisotropy of the film.
It is generally desirable to achieve small grain sizes in ferrite thin films in order to provide media useful for ultra-high density magnetic recording applications. In order to achieve a 10 Gbit/in.sup.2 recording density, it is generally considered that a grain size of the order of 100 .ANG. is necessary. However, the minimum grain size suitable for recording purposes will be limited by the thermal stability of the magnetic properties of the individual grains.
Ferrite thin films can be fabricated by sputter depositing ferrite materials onto various types of substrates. The films are amorphous and nonmagnetic as sputter deposited at room temperature, and must be converted to a magnetic form for use as magnetic recording media, in microwave and millimeter wave devices, and in various other technologies. The amorphous films may be caused to undergo an amorphous-to-crystalline phase transformation to a magnetic film by subjecting them to either an in-situ or ex-situ annealing. See A. Morisako et al., "Influences of Sputtering Gas Pressure on Microstructure and Crystallographic Characteristic of Ba-Ferrite Thin Films for High Density Recording Media", IEEE Trans. Magn. 23, 56 (January 1987); P. Gerard et al., "Crystallization in Thin Films of Amorphous Barium Hexaferrite", Solid State Comm. 71, 57 (1989); X. Sui and M. Kryder, "Magnetic Easy Axis Randomly In-Plane Oriented Barium Hexaferrite Thin Film Media", Appl. Phys. Lett. 63, 1582 (September 1993). In the in-situ annealing process, the films are annealed during deposition, and the crystalline form of ferrite forms as the film is laid down. An ex-situ, or "post-deposition", annealing process is one applied to films already deposited in their amorphous and nonmagnetic form, and the films only become crystalline and magnetic when annealed under appropriate conditions.
The annealing temperatures necessary to produce crystalline ferrite thin films are very high, typically around 600.degree. C. for in-situ annealing and around 800.degree. C. for ex-situ annealing. At these high temperatures the atoms in the substrate materials may diffuse a significant distance into an overlying ferrite thin film. For example, T. Hylton et al., "Ba-Ferrite Thin-Film Media for High-Density Longitudinal Recording", J. Appl. Phys. 75, 5960 (May 15, 1994), reports the presence of a 260 .ANG. non-magnetic interdiffused layer at the interface of a 500 .ANG. Cr.sub.2 O.sub.3 -doped barium ferrite thin film on an oxidized silicon substrate. It is believed that within or even near the diffused layer in the thin film, traditionally referred to as the "dead layer", grains of undesirable crystallographic orientations, of chemical compositions other than the ferrite material, or of non-uniform sizes may be formed as a result of the presence of the diffused substrate atoms. In a sense, the dead layer is not truly dead, but may contain some amount of magnetically active grains. Because of the deviations in crystal orientation, chemical composition, and grain size in the dead layer, that layer's magnetic properties may be deteriorated.
In the case of ferrite thin film magnetic recording media, the deterioration of the dead layer's magnetic properties and its constituent grains' non-uniform composition, orientation, and size may significantly degrade the magnetic recording properties of the thin films. For example, grains of perpendicular orientation may reduce the in-plane M.sub.r and H.sub.c of the film. Grains of larger sizes typically exhibit lower H.sub.c due to incoherent rotation of magnetization. Grains consisting of atoms other than those of the film's constituent ferrite material may also have different anisotropy and, consequently, different H.sub.c. Films incorporating two or more phases (including differing compositions, orientations, or grain sizes) generally would be expected to have lower S* values than a film of a single phase.
The presence of a dead layer may likewise adversely affect magnetic ferrite thin films used in other applications. For example, the presence of a dead layer in a ferrite thin film used in microwave applications could adversely affect the homogeneity of the film, causing dispersion in the magnetic resonance performance of the devices.
Many types of high temperature-durable substrates have been tested and found unsuitable for the direct application of ferrite thin films because of the problems resulting from diffusion. In particular, a body of work devoted to identifying useful barium ferrite thin film recording media has identified a number of substrates that will diffuse into an overlying barium ferrite thin film layer at high temperature. See E. Lacroix et al., "Substrate Effects on the Crystalline Orientation of Barium Hexaferrite Films", J. Appl. Phys. 69,4770 (Apr. 15, 1991). Various underlayers (also referred to as boundary layers), disposed between the substrate and a ferrite thin film layer, have been attempted to prevent diffusion from the substrate. One commonly investigated underlayer type is the oxides. For example, P. Dorsey et al., "Oriented Barium Hexaferrite Films Grown on Amorphous Substrates", J. Magn. Magn. Mater. 137, 89 (1994), discloses a zinc oxide underlayer deposited on a fused quartz substrate using a pulsed laser deposition technique at 600.degree. C. A barium ferrite layer was deposited on the zinc oxide underlayer at 750-800.degree. C. and exhibited a c-axis orientation. A considerable amount of zinc diffused from the underlayer into the magnetic layer, and the composition of the thin film at the surface was barium ferrite (BaFe.sub.12 O.sub.19) with approximately 3% zinc substituted for iron.
A sputtered silicon dioxide boundary layer deposited on a carbon substrate is taught in K. Sin et al., J. Appl. Phys. 73, 6689 (1993).
Other variations of underlayers intended to prevent the adverse effects of substrate atom diffusion include silicon nitride coated onto a carbon substrate. A randomly oriented crystalline barium ferrite thin film was then deposited in-situ onto the silicon nitride layer by facing target sputtering at a substrate temperature of 650.degree. C. without post-deposition annealing. J. Li et al., "High Density Recording Characteristics of Sputtered Barium Ferrite Thin Films", IEEE Trans. Magn. 31, 2749 (November 1995).
All of these known underlayers are not entirely satisfactory because they may introduce into an overlying ferrite thin film a diffused layer containing magnetic grains nearly as undesirable as the grains produced by the diffusion of substrate atoms directly into the ferrite thin film layer.
Noble metals have also been investigated as underlayer boundaries to diffusion in ferrite thin films. For example, X. Sui, "Growth of Perpendicular Barium Hexaferrite Thin Film Media on a Pt Underlayer for High Density Perpendicular Magnetic Recording", J. Magn. Soc. Jpn. 18, S1, 19 (1994), discloses the use of a sputtered 2500 .ANG. thick platinum underlayer between a thermally oxidized silicon substrate and a sputtered 600 .ANG. thick barium ferrite layer. The deposited layers were annealed ex-situ at 800.degree. C. in air to bring about the amorphous-to-crystalline transition in the barium ferrite layer.
Accordingly, in order to address the demand for better magnetic ferrite thin films, the need remains for a ferrite thin film construction that will inhibit the detrimental affects to the magnetic ferrite thin film layer resulting from the diffusion of substrate atoms at high temperatures. In particular, to address the demand for improved magnetic information storage devices, a need exists for ferrite thin film recording media of a construction that will inhibit the detrimental magnetic affects of substrate atom diffusion during annealing.